In many printing systems, sheets are fed from a feeder, registered with a paper registration system before it receives the image. The image is then fused onto the first side of the paper. The sheet is then inverted and routed to through the duplex path to present the second side of the paper to the imaging device.
Generally, the sheet registration device prepares the sheet for delivering to the datum and a target time. The preparation includes alignment of the sheet and time adjustments of delivery. Sensors measure the lateral and skew of the sheet. Adjustments to the sheet are made accordingly. At the same time, the sheet passes over a lead edge sensor which records or measures the sheet's position when it arrives at the registration system. This information is used to deliver the sheet at a target time to the registration datum (i.e. transfer) for imaging.
In a duplex system, different systems are used to deliver the inverted sheet to the datum at the target time. One system measures the lead edge on the first side and second side in the same manner using a lead edge sensor. The lead edge sensor records the time of arrival to the lead edge of the sheet. The arrival information is used to deliver the sheet at a target time to the datum regardless of which side of the sheet is being introduced at the time. However, sheet registration based on only the leading edge of the sheet regardless of inversion is inaccurate due to the sheet property variations of each side and edges of the paper. This system does not take into account a differentiation between the leading edge and trailing edge of a sheet and variations in the sheet sides.
Another system used to deliver a sheet to the datum at the target time is measuring the lead edge for one side and a trail edge for the inverted side. As above-discussed the first side of the sheet uses sensors to measure the lateral and skew and adjustments are made. The leading edge sensor is used to record the time of arrival of the leading edge of the first side of the sheet. The sheet is then delivered to the datum at the target time. After the image is fused onto the first side of the sheet, the sheet is inverted and sent back to the registration system for imaging on the second side. The registration system uses the sensors to measure lateral and skew, as similarly measured for the first side of the sheet. The sheet is adjusted accordingly. While the second side of the sheet is measured and adjusted, the trail edge sensor records the time of arrival of the trail edge of the second sheet (which is also the leading edge of the first side of the sheet). However, the trail edge sensors record inaccurate data because the sheet is being adjusted and moved during recordation of the trail edge.
Therefore, there is a desire for a system that provides accurate recordation and measurement of both sides of the sheet to deliver the sheet to the datum at the target time. Additionally, the system must be flexible enough to provide for accurate delivery of sheets that vary in size and other properties. Further, there is a desire to provide a system for providing accurate delivery to the datum that can compensate for variations and delaying within the system itself.